mineralogy of the lunar crust: results from clementine · mineralogy of the lunar crust: results...

Meteoritics & Planetary Science 34, 2 5 4 1 (1999) 0 Meteontical Society, 1999 Printed in USA

Mineralogy of the lunar crust: Results from Clementine STEFANIE TOMPKINSI* AND CARLE M. PIETERS2

'Science Applications International Corporation, 4501 Daly Drive, Suite 400, Chantilly, Virginia 201 5 1, USA 2Department of Geological Sciences, Brown University, Providence, Rhode Island 029 12, USA

*Correspondence author's e-mail address: [email protected]

(Received 1997 December 5; accepted in revised form 1998 September 15)

Abstract-The central peaks of 109 impact craters across the Moon are examined with Clementine ultraviolet-visible (UVVIS) camera multispectral data. The craters range in diameter from 40 to 180 km and are believed to have exhumed material from 5-30 km beneath the surface to form the peaks, including both upper and lower crustal rocks depending on whether craters have impacted into highlands or basins. Representative five-color spectra from spectrally and spatially distinct areas within the peaks are classified using spectral parameters, including "key ratio" (which is related to mafic mineral abundance) and "spectral curvature" (linked to absorption band shape, which distinguishes between low- and high-Ca pyroxene and olivine). The spectral parameters are correlated to mineralogical abundances, related in turn to highland plutonic rock compositions. The derived rock compositions for the various central peaks are presented in a global map. From these results, it is evident that the lunar crust is compositionally diverse, both globally and at local 100 m scales found within individual sets of central peaks. Although the central peaks compositions imply a crust that is generally consistent with previous models of crustal structure, they also indicate a more anorthositic crust than generally assumed, with a bulk plagioclase content of -8 1%, evolving from "pure" anorthosite near the surface towards more mafic, low-Ca pyroxene-rich compositions with depth (comparable to anorthositic norite). Evidence for mafic plutons occurs in both highlands and basins and represent all mafic highland rock types. However, the lower crust is more compositionally diverse than the highlands, with both a greater range of rock types and more diversity within individual sets of central peaks.

INTRODUCTION

Lunar crustal evolution is broadly understood within the framework of a magma ocean model, in which the Moon's ancient crust formed through the flotation of anorthite-rich plagioclase in a global fractionation event ( e . g , Warren, 1985; Warren and Wasson, 1977; Wood et al., 1970). Other lithologies such as magnesian-suite rocks and mare basalts are believed to have formed through subsequent serial magmatism. These models satisfy constraints imposed by a variety of data, including lunar sample chemistry, Apollo gamma-ray and x-ray spectrometry, and geophysical measurements. These data, however, are limited by either spatial resolution or a lack of global coverage. In particular, the lunar sample collection is a relatively small and non-random representation of the Moon's crust, yet the presence and abundance of specific lithologies (the fenoan anorthosite suite, low-K Fra Mauro (LKFM) rocks, the magnesian suite, etc.) within the collection impose key constraints upon models for both crustal evolution and the current composition and structure of the lunar crust.

Remote sensing provides a global perspective that the lunar sample collection alone cannot. The Clementine mission, which returned global, high-spatial resolution multispectral images of the lunar surface, presents an opportunity to examine compositional trends across the entire Moon. For example, Clementine spectral data have been used to estimate FeO and Ti02 abundance of the weathered soils that blanket the Moon's surface (e.g., Lucey et al., 1995, 1998). For more pristine rocks and fresh soils, whose optical properties have not been extensively altered by space weathering processes, mafic mineralogy may be identified (Pieters et al., 1994, 1997; Tompkins and Pieters, 1997). Where such materials have been uplifted from beneath the surface, as in the central peaks or peak rings of impact craters, identifying their mineralogy offers a glimpse of crustal composition at depth.

In this paper, the results of a compositional survey of the central peaks of 109 impact craters are presented. The craters are globally distributed, representing crustal material from 5-30 km beneath the surface. Although some of the craters are in areas of mare-basalt fill, all of the central peaks appear to be nonmare rocks and are assumed to have been exhumed from beneath the basalt layers. In addition to providing a spatial context (both vertical and lateral) for the lunar sample collection, the central peak compositions make up a statistically significant database with which to consider the structure and composition of the lunar crust.

BACKGROUND Remote Measurements

From visible through near-infrared (NIR) wavelengths, crystal field transitions of Fe and Ti cause diagnostic absorption bands in reflectance spectra that allow mafic minerals such as olivine and pyroxene to be identified remotely in lunar materials. Over the past 20 years, a vast database of telescopic reflectance spectra has been acquired for locations across the lunar nearside (e.g. , McCord et al., 1981; Pieters, 1993). Spectra of impact craters have been particularly fruitful, as craters expose fresh material from beneath the surface; and in the case of complex craters, uplift deep-seated materials from kilometer-scale depths beneath the surface. The spectral data indicate that the megaregolith, exposed in the floors of small craters and the walls of larger ones, is predominantly composed of noritic anorthosite (or brecciated rocks and soils of that composition) (Pieters, 1986). At greater depths, either beneath or near the bottom of the megaregolith, a larger compositional range is found, with examples of all of the major highland plutonic rock types exposed in the central peaks of complex craters (e.g., Hawke et al., 1986; Lucey et al., 1986; Pieters, 1991, 1993; Pieters and Wilhelms, 1985). A summary of the compositional stratigraphy of the nearside lunar crust derived from limited Earth-based NIR spectra of the walls, rims, and central peaks of large craters is shown in Fig. 1 (after Pieters, 1993).

25

26 S. Tompkins and C. M. Pieters

Anorthosite

Norite

Gabbro

Troctolite

Luna

FIG. 1. Summary of compositional stratigraphy of the lunar highland crust as exposed by the rims, walls, and central peaks of large impact craters and determined from NIR telescopic spectra (Pieters, 1993). Smaller craters, which exhume material from within (rather than beneath) the megaregolith, indicate that the upper 5 km of the crust has a composition that is largely equivalent to a noritic anorthosite (Pieters, 1986).

Although the telescopic measurements have high-spectral resolution that allows detailed assessments of mineralogy, only major spatial variations in composition can be detected with these spectra because they represent areas 2-10 km in diameter. In contrast, Clementine multispectral images offer increased spatial resolution but significantly less spectral information. In 1994, the Clementine ultraviolet-visible (UVVIS) camera acquired global multispectral images with pixel resolutions of 100-300 m, for five spectral channels between 0.4 and 1.0 pm. These images have already been used to examine compositions at several impact craters (e.g. , McEwen et al., 1994; Pieters et al., 1994) and to estimate the global distribution of surface Fe (Lucey et al., 1995). But direct comparison of mineral absorption bands between locations across the Moon could not be accomplished until recently, when spectral and photometric calibration methods of the data were more complete.

Lunar Sample Context Roughly half of the pristine rocks in the lunar highland sample

collection are part of the ferroan anorthosite suite, which generally contain >85% anorthite-rich plagioclase (e.g. , Heiken et al., 1991). These rocks are widely believed to represent the plagioclase-rich portion of a massive differentiation event occurring early in the Moon's history (Walker et al., 1975; Warren and Wasson, 1977). The most common model for this event invokes a global magma ocean, but alternative models have been suggested. Instead of a completely molten ocean, for example, serial magmatism models involve localized differentiation, either in the form of multiple near- surface plutons or rising diapirs of plagioclase from deeper zones of localized melting (e.g. , Longhi and Ashwal, 1985; Walker, 1983). Mapping the global occurrence of plagioclase on the Moon with remote sensing data provides a much needed spatial context for the lunar sample collection. Identification of local areas of mafic-free plagioclase has been (and continues to be) accomplished using high- spectral resolution NIR spectra acquired with Earth-based telescopes

(Hawke et al., 1991, 1995; Peterson et al., 1996; Pieters, 1986, 1993; Spudis et al., 1984, 1988, 1989). The Clementine data supplements these studies with higher spatial resolution and by the inclusion of areas not observable from Earth-based telescopes.

Most of the remaining pristine highland rocks exhibit a greater range of plagioclase abundance and composition as well as more abundant and more Mg-rich mafic silicates than do the anorthosites. The lunar samples of the magnesian suite are cumulates-most commonly noritic or troctolitic-and are generally believed to have formed after the Moon's primary crustal differentiation was complete. Various authors have suggested that the magnesian suite rocks formed as layered mafic plutons (e.g., Warren and Wasson, 1980) that intruded into the already formed anorthositic crust. They are chemically too distinct to have formed from the same magma as the anorthosites (and despite an overlap in crystallization ages) are also generally younger (e.g. , Nyquist and Shih, 1992). Although the value of Mg/(Mg + Fe) of a rock cannot be determined with the type of data available from Clementine, candidate magnesian suite rocks may be identified by the greater abundance of mafic minerals when compared to the anorthosites and by the greater abundance of plagioclase when compared to volcanic materials such as mare basalts.

The central peaks data can be used not only to test the completeness of the lunar sample collection directly but also to test predictions of crustal composition that are derived from the lunar samples. For example, using the compositions of pristine lunar samples, variations on the magma ocean model have been used by various authors to predict constraints on crustal composition. Warren and Kallemeyn (1 993) predict that 30 wt% of the lunar crust is of rnagnesian-suite composition. Warren (1990) has also suggested that mafic-rich anorthosite (15 wt% mafic mineral content) is globally common. Predictions such as these can be tested directly with the mineralogy, frequency of occurrence, and distribution of rock types observed among the surveyed central peaks.

DATA COLLECTION AND PROCESSING

Crater Selection

Although there remains serious discussion regarding the mecha- nisms by which central peaks are uplifted and the exact nature of the uplift depth-to-crater-diameter relationship, simple estimates are sufficient for the purposes of our survey. Scaling relationships from terrestrial craters indicate that the preimpact depth of lunar central peak material can be related to the final crater diameter; and that for craters between 40 and 150 km in diameter, the excavation depth is between -5 and 30 km (Cintala and Grieve, 1994). Craters included in this survey were initially selected with the simple criteria that their diameters be >40 km and their central peaks sloped steeply enough to prevent accumulation of a heavily altered soil.

Although nearly 200 craters were initially selected, data avail- ability limited the final number. For consistency, the central peaks were required to have a spatial resolution of better than 240 dpixe l (most have 100-150 dpixel) and to fall within a single Clementine orbit (to eliminate color variations due to viewing geometry that occur between orbits). Furthermore, there are a number of orbits where there is no data for one or another of the five UVVIS camera filters; although this factor alone did not eliminate a crater, a missing 0.75 p m filter did, as most spectral interpretations rely on knowledge of the reflectance near that wavelength. The 109 craters finally selected are listed in Table 1.

Mineralogy of the lunar crust: Results from Clementine 27

TABLE 1. List of impact craters for which the central peaks compositions have been identified, along with compositions.

Crater Latitude Longitude Diameter Age Setting Peak lithologies (km)

Clementine filename

Aitken Alphonsus Appleton Aristarchus Aristillus Aristoteles Arzachel Atlas Ball Barringer Belyaer Berkner Bernoulli Bettinus Bhabha Birkeland Borman Bose Bridgman Bullialdus Burg Campanus Cantor

Carpenter Compton Copernicus Crookes

Cyri I I us Daedalus Delporte Doppel-mayer Eichstadt Einstein

Eratosthenes Eudoxus Fabricius Finsen Fizeau Gassendi Hahn Hale Hayn

Helmholtz Hercules

Hubble (Plutarch A) Jackson

Joliot-Curie Keeler King

Kirkwood Kovalev-skaya La PCrouse Langmuir Langrenus

Lansberg Leavitt Leuschner Lindenau

17" S 14" S

40" N 23 5" N 34" N 50" N 1 8 5 " s 47" N 36" S 28" S 23" N 25" N 35" N 64" S

55 5" s 30" S 39" s 55" s

43 5" N 21" s

45 5" N 28" S

38 5"N

70" N 56" N 10" N

10 5's

13 5" S 05 5" S

16" S 28 5" S 22" s 17" N

14 5"N 57 5" N 43" s

42 5's 58 5" S 17 5 " s 31 5"N

74" s 64 5" N

69" S 47" N

22 5" N 22O N

26" N 10" s 06" N

69" N 31" N 10 5" s 36 5" S 08 5" s 01"s 46" S

01 5"N 33" s

173.5" 357" 158"

312.5" 1.5" 17'

358" 39"

351.5" 209" 143" 255" 61" 315" 195" 174" 211"

190.5" 137" 338" 28.5" 332" 119"

309.5" 105" 340" 195'

24" 180"

121.5" 3 19" 282" 288"

348.5" 16.5" 42"

181.5" 224.5" 320" 74" 90" 84"

65.5" 3 9"

87" 197"

93.5" 162"

120.5"

123" 230.5"

71" 231" 61'

333.5" 219.5' 251" 25"

118 119

40 55 87 97 87 40 69 51 95 47 82 78 85 55 94 94 61 40 48 75

60 162 98 50

93 100 40 65 45 48

58 67 78 87 110 110 84 85 87

110 68

82 71

143 132 71

70 109 80 85 132

40 80 42 55

UI N N C C E LI UI

unknown N

unknown unknown

UI unknown unknown

E LI

unknown unknown

u1 C LI UI

C LI C C

N L1

unknown N

unknown unknown

E C E E

unknown N LI

unknown C

unknown E

N C

PN LI C

E UI

unknown unknown

E

UI unknown unknown unknown

highland basin highland basin basin basin basin highland highland basin basin highland highland basin basin basin highland basin highland basin basin basin highland

highland highland basin highland

basin highland highland basin basin highland

basin highland highland basin highland basin highland highland highland

highland highland

highland highland

highland highland highland

highland highland highland highland basin

basin highland highland highland

A, GNTAI, GNTA2 A A GNTAl, GNTA2 GNTA2, AN A, GNTA2, AN GNTAI, GNTA2 GNTAl, AG GNTA 1 GNTAl , GNTA2 A, GNTAI A, GNTAl A, GNTAl A, GNTAl AGN, AN, GN, G GNTA2, AN, GN GNTA2 GNTAZ, AGN, AN A, GNTA I GNTAl, GNTA2, AN, N GNTA2, AN GNTAl, GNTA2, AN A, GNTAl

A, GNTAI A, GNTAl , GNTA2 GNTAI ,GNTA2, AT GNTAl ,GNTA2, AT

A A A A A, GNTAI A

A, GNTA I , GNTA2, AGN A, GNTA I , GNTA2 A,GNTA I , GNTA2 GNTA2, AGN, AN, GN, N GNTA2, AN GNTAl A, GNTAl GNTAI, AN GNTAl, GNTA2

A GNTAI

A, GNTAl GNTAl, GNTA2, AGN

A, GNTAl A, AT GNTA2, AGN

A A, GNTA1, GNTA2 GNTAI, GNTA2 A, GNTAl, GNTA2, AGN GNTAI, AN, T

GNTAl, GNTA2, AGN A, GNTAl, GNTA2 A, GNTAl A, GNTAl , GNTA2

lub1490h.237 lub2746h.037 lub4870m.llO Iub29901.186 lub3372m.168 lub4489n.294 lub2560h.037 lub5 146n. 152 tub181 5f.039 lub2185g.091 tub31 101.248 lub32641.207 lub3646m.278 I ub0943 c. 05 2 lub1266d.096 lub2343g. 104 lub195 1 f.090 tub I295d.098 lub3861n.250 lub2774g.044 lub4115n.290 lub2460g.046 lub3740m.257

no 1.Opm image lub6202p.3 18 lub43020.262 lub3706j.043 lub 176911.229

no 1 .O p n image lub2969h.159 lub3343i.102 lub1340h.256 lub1694g. 183 lub 1469g. I 97 lub3921 k.333

no 1 . O i im image lub2 177k.305 lu b4225n.294 lub1829e.152 lub1482e.101 lub0900d.085 Iub285Oh. 183 lub4618m. 141 lub0616b.134 lub4829p.270

no 1 . O itm image lub0645c.143 lub4113n.286 lub4146n.286 lub44991.136 lub30471.228 lub30781.228 lub29771.266 lubl831 h.241 tub208 1 j.256 lub2112j.256

no 1 .Opn image lub4768p.226 lub3270m.216 lub3285h.140 lub 1845f.083 lub1757i.278

no 0.95 p n image lub3639i.046 lub1457e.087 lub3734j.076 lub1427f.027

no 1 . O p image


TABLE 1. Continued.

Crater

Lobachev-sky Lodygin Lowell Lyman Mach Manilius Maunder Maurolycus Mendell Miller Millikan

Morse Nansen Neper Newcomb O'Day Ohm

Olcott

Orlov

Paracelsus Philolaus Piccolomini Pitatus

Pitiscus Plana Plinius Plutarch Posidonius Pythagoras Robertson

Rydberg Scaliger Schjellerup Schorr Scoresby Sharonov Slipher Stevinus Stiborius Theophilus Tsiolkovsky

Tycho unknown in Korolev unknown W.

of Millikan Vavilov Vitello Vlacq Von Neumann White Wiener Zucchius

Latitude Longitude

-

09.5" N 18" s 13" S 67" N 18"N

14.5" N 14.5" S 42" S 51" S 39" s 47" N

22" N 81"N 09" N 30" N 31" S l8"N

20.5" N

26" S

23" S 72.5" N 29.5" S 30" S

51" S 42" N 15.5" N 24.5" N 32" N 64" N

21.5" N

47" s 27" S 70" N 19.5" S 77.5" N 12" N 50" N 33" s 35" s

1 1 So S 20" s 43" s 09" S 46" N

01.5" S 30.5" S 53.5" s 40.5" N 46" S 41'N 61" S

113" 213" 257" 162" 21 I D

9" 266" 13.5" 250"

I " 121"

185" 95" 85" 44" 157" 246"

118"

185'

163" 327.5"

32" 346.5"

31" 28.5" 23.5" 79" 30"

297" 255"

264' 109" 157" 90" 14"

173.5" 160" 54.5O 32" 26" 129"

349" 203" 118"

221" 323.5"

39" 153" 190" 146" 310"

Diameter (km)

62 50 66 90 155 40 55 137 160 68 100

62 116 139 40 61 64

67

61

71 71 88 101

90 44 43 68 95 130 88

50 78 68 48 56 65 87 75 45 100 185

85 57 53

99 42 98 95 45 131 64

UI unknown

C UI

unknown E E

unknown unknown unknown

N

E N N E C C

E

unknown

unknown C UI N

unknown unknown

E E UI E C

E UI UI UI E C UI C

unknown E

UI

C N

unknown

C LI

unknown UI

unknown N C

Setting Peak lithologies Clementine filename

highland highland basin highland highland basin basin highland highland basin highland

highland highland basin highland basin highland

highland

highland

highland highland basin basin

highland basin basin highland basin highland highland

highland highland highland highland highland highland basin highland highland basin highland

highland basin highland

highland basin highland highland highland highland basin

GNTAI, GNTA2 A GNTA1, GNTA2, AG AN, N A A GNTAI, GNTA2, AG A A, GNTA2 A, GNTAl A. GNTAl

A, GNTAl A A, GNTAl, GNTA2 GNTAI, GNTA2 A, GNTAl, GNTA2 GNTA 1, GNTA2, AG

A, GNTAI, GNTA2

A, GNTAI, GNTA2, AGN

A, GNTAl A, GNTAl A, GNTAl A, GNTAl

A, GNTAI, GNTA2 A, GNTAl A A, GNTAl A A, GNTAl GNTAI. GNTA2

A A, GNTA 1, GNTA2, AG A, GNTAl A, GNTAl GNTAl A, GNTAI, GNTA2 A, GNTAl GNTA2, AG, AGN, AN A, GNTAl A, G N T A l , GNTA2, A T A, G N T A l ,GNTA2, AT

GNTA2, AG, AGN, G A GNTAl

A, GNTAl, GNTA2, AGN GNTAl, GNTA2 A GNTAl GNTA2, AG, AN, GN A, GNTAl GNTAl, GNTA2, AG, G

lub2505j.259 lub1355h.222 lub3 145h.074 lub0802c.108 lub2956k.223 Iub2798k.297 lub 1685h.203 lub0889e.295 lubl564e.076 Iub0707f 168 lub3879n.256

no 1 .O pm image lub43681.100 lub5178r.266 lub2475j ,269 Iub47121. I52 lub2328f.110 Iub2701 k.2 10 lub2732k.210 lub29961.257

no 1 . 0 p n image lub2476g.100 Iub2507g. I00 lub2538g. 108 lub4985q. 180 lub243 1 g. I56 lub2063f 041 lub2094g.041 lubl50ld.l56 Iub3983n.290 lub4085k.159 lub44731.139 lub3803m.289 Iub5027p.059 lub3 1091.207 lub3 1401.207 lub 1380e.07 1 lub2539g.128 lub5086p.243 lub2691 h. 135 lub5542q.295 lub2699k.237 lub 1356h.265 lub2264f I48 lub22 l4f. I56

lub3237h.158 lub1420g.253 lub145 lh.253 lub1781e.040 lubl605i.226 lub4081n.257

no l . O p n image lub3380i.087 lub0955q.182 lub0450d.286 lub4902n. 112 lub I770e.094 lub4956n.114 lub1067c.054

Age: Approximate crater ages are presented where available. Abbreviations: PN = Pre-Nectarian, N = Nectarian, LI = Lower Imbrian, U1= Upper Imbrian, E = Eratosthenian, and C = Copernican. Setting: As discussed in the text, craters are subdivided based on whether they occur in the highlands or within or near major impact basins. Peak lithologies: Rock-type abbreviations: A = anorthosite, GNTAI = gabbroic-noritic-troctolitic anorthosite with 85-90% plagioclase, GNTA2 = gabbroic- noritic-troctolitic anorthosite with 8&85% plagiocalse, AN = anorthositic norite, AGN = anorthositic gabbronorite, AG = anorthositic gabbro, AT = anorthositic troctolite, N = norite, GN = gabbronorite, G = gabbro, and T = troctolite. Clementine filename: The Clementine filename for the 750-nm image(s) used for each crater.


Image Calibration

An overview of the Clementine mission and measurement strategy can be found in McEwen and Robinson (1997). The calibration procedures used on the UVVIS camera images are outlined below. Images were flatfield and darkfield corrected (including various instrumental offsets), and photometrically adjusted to a standard viewing geometry of 30" phase (i = 30°, e = 0'). For each image, a single photometric correction factor was calculated based on the viewing geometry at the image center. Where necessary at high-phase angles, an empirically-derived correction factor was applied to the 0.4 p m images to mitigate the effects of a wavelength-dependent photometric function (Nygard, 1975). The images were calibrated to reflectance using laboratory measurements of an Apollo 16 soil sample as described in Pieters et al. (1996b). Finally, the calibrated images were co-registered to the nearest 0.1 pixel in order to create color ratio images and plot reflectance spectra. Most of the central peaks fall within a single image frame; but where necessary, two or more images (within a single orbit) were mosaicked together.

It is noted that the normal photometric corrections used appear to overcorrect our central peak data at very high-phase angles. Re- flectance on some surfaces appears -20% brighter than that of any known lunar materials, even after estimating errors due to topographic effects. The excessive brightness for steeply sloped surfaces near the poles is probably an uncorrected nuance of the lunar photometric function. The excessive brightness does not appear to be wavelength dependent, so normalized spectra may be directly compared to normalized spectra acquired at smaller phase angles. In general, however, compositional interpretations linked to albedo for craters near the poles (greater than k60" latitude) are tenuous, and such craters are therefore clearly indicated in this paper for most figures involving spectral parameters.

Like all datasets, Clementine UVVIS data have their idiosyn- crasies. Although calibrations currently approach I% accuracy (McEwen and Robinson, 1997), there remain some problems, largely due to scattered light, and particularly affecting the 0.95 p m band, which is key to interpreting mafic mineralogy. Problems were minimized by using average spectra and by careful inspection of each of the images to extract representative spectra. Admittedly, a great deal of the error minimization was based upon "feel" for the spectra; spectra without smooth continuous absorption bands indicated poor quality data. Where possible, alternate orbits containing the same area were found. If duplicate data did not exist for ambiguous areas, the crater in question was excluded from the survey.

DATA ANALYSIS AND INTERPRETATION Optical Properties

The most common rock-forming minerals on the Moon may be identified remotely through the analysis of reflectance spectra (e.g. , McCord and Adams, 1973). Iron-bearing silicate minerals such as pyroxene and olivine have characteristic Fe2+ electronic transition absorption bands at NIR wavelengths as shown in Fig. 2 (e.g., Burns, 1993). The position of absorption band minima-as well as the band shape and strength-are all indicative of a mineral's crystal structure and composition and are thus diagnostic of the dominant mafic mineral present. Pyroxenes have absorption bands near both I and 2 pm. Adams (1974), Hazen et al. (1978), and Cloutis and Gaffey ( 1 991) have demonstrated that the precise wavelength posi- tions of these band minima are indicators of pyroxene chemistry, with the minima moving to longer wavelengths with increasing Ca

0.7

0.6

0.5 m C

0.4 0) - - ?

8 0.3

0.2

0.1

0

i

0 500 1000 1500 2000 2500 3000 wavelength (nm)

FIG. 2. Laboratory spectra of lunar minerals with Clementine bandpasses superimposed. All samples are particulate, with particle size 2250 pm. Lunar samples: 15415 (anorthite), 12063 (high-Ca pyroxene), 78235 (low- Ca pyroxene), and 72415 (olivine).

and (to a lesser extent) Fe. On the other hand, olivine has three overlapping absorption bands near 1 p m (which may appear as one broad band) and none near 2 p m (e.g., Burns, 1970; Sunshine and Pieters, 1998). For rocks and soils containing olivine and/or pyroxene, band strength is generally proportional to the abundance of these minerals, their particle size, and (in some cases) their Fe2+ content. However, the presence of strongly absorbing species (opaque minerals such as ilmenite) or the process of space weathering can weaken band strength. Therefore, although a strong absorption band clearly indicates a high abundance of Fe2+-bearing silicates, a weak band may have multiple causes.

Plagioclase feldspar, the most abundant mineral in lunar highland rocks, has a weak absorption band near 1.3 p m when very small concentrations (<0.5 WWO) of Fe are present (Adams and Goulland, 1978). Although this feature is commonly diminished or lost entirely through shock processes (Adams et al., 1979), plagioclase also has a higher reflectance relative to mafic minerals at all visible-to-near- infrared wavelengths, so its presence can be inferred from the relative brightness of a given spectrum. Furthermore, because abundant plagioclase in lunar highland rocks generally implies a smaller concentration of mafic minerals, weak mafic absorption bands in a high albedo matrix also imply the presence of abundant plagioclase. However, as stated above, weak absorption bands have multiple causes and must always be considered in conjunction with other, more diagnostic spectral properties. With telescopic NIR data, detection of anorthosite (or mafic-free plagioclase) relies on a characteristic high albedo and lack of detectable absorptions due to mafic minerals.

The mineral spectra in Fig. 2 exhibit the diagnostic features by which lunar rock compositions may be identified remotely. Shown in Fig. 3 are the same spectra convolved through the five Clementine UVVIS camera filters. Diagnostic distinctions are less prominent, although the four minerals are still readily distinguished from one another. However, lunar rocks and soils observed in the Clementine images are more likely to represent mixtures of these minerals, and actual spectral variations are therefore subtle. A small calibration error from one filter to another can completely change the compositional interpretation (e.g. , distinguishing high-Ca pyroxene

30

:"i;,. 20 i 1 0

S. Tompkins and C. M. Pieters

Lunar Mlnerals through Clementlne Filters 70 , 7 7 1 I - - ) , 1 1 8 a . 1 8

e a n o r t h i t e - low-Ca pyroxene -olivine

0 " " " " " " " " "" " " ""

400 500 600 700 800 900 1000 1100 wavelength ( n m )

FIG. 3. Lunar mineral spectra of Fig. 2 convolved through Clementine filters. Note the change in wavelength scale. The minerals may be distinguished in the five-color spectra, but spectral differences are more subtle.

from olivine depends strongly on the relative reflectance between 0.95 and 1 .O pm).

To test calibration and mineral interpretations of the five-color data, high-quality NIR telescopic spectra were convolved through the Clementine filters for three craters for which high-spectral resolution telescopic measurements exist (Tycho, Bullialdus, and Copernicus). These modified telescopic data are compared to similar five-color Clementine spectra in Fig. 4. (A correction factor was applied to the telescopic spectra4alibrated to directional-hemispheric reflectance geometry-to make them comparable to the bi-directional reflectance of Clementine.) The independent data agree quite well and the pseudo-Clementine telescopic spectra retain the mineralogical signature interpreted from the high-resolution spectra. For a closer look, in Fig. 5 the telescope-based spectrum for Bullialdus' peaks is plotted with a Clementine spectrum for an area that approximately matches that seen through the telescope (Pieters, 1991).

Optical alteration associated with space weathering can further complicate the process of spectral interpretation. Exposure to processes in the space environment-such as micrometeorite bom- bardment and solar wind implantation-alters lunar materials and forms a regolith containing complex soil particles. Although central peaks are not immune to the effects of space weathering, their topographic relief typically minimizes the development of fully mature soils on their slopes. An assumption of the central peaks survey is that the peaks have relatively fresh surfaces (compared to flat-lying crater floor or ejecta material), and their exposed surfaces are sufficiently similar in maturity for simple comparisons. Never- theless, although all of the craters are used in this survey to examine global trends, direct correlation between spectra of craters of different ages should not be done indiscriminately. Future work will focus upon quantifying the extent to which such comparisons can be made.

Several recent lunar spectroscopy studies have focused on under- standing optical alteration induced by space weathering (Fischer and Pieters, 1994; Pieters et al., 1993), and a few Clementine analyses have developed spectral parameters using reflectance only at 0.75 and 0.95 p m to generally decouple the effects of maturity to estimate FeO abundance (Fischer and Pieters, 1996; Lucey ef a)., 1995). However, these simple parameters do not provide usehl estimates of composition for central peaks because they depend on accurate measurements

of absolute reflectance. Both dramatic variation of illumination geometry and grain size are uncontrolled variables and strongly affect the measured reflectance values of the steep-sided peaks.

Image AnalysidSelection of Spectra

We analyzed the Clementine data in two stages: as images for spatial information and as spectra for mineral content. After calibration, a set of image ratios was created in order to examine spatial variations across the central peaks, consisting of 0.415 p d 0 . 7 5 p m (sensitive to relative freshness) (e.g., Fischer and Pieters, 1994), and the three "mafic ratios," 0.75 p d 0 . 9 0 pm, 0.75 p d 0 . 9 5 pm, 0.75 p d 1 . 0 pm (sensitive to the strength and position of the 1 p m absorption band). Any one of the latter three provides a first-order assessment of the proportion and distribution of mafic materials across an image. The advantage of this type of image analysis lies in ascertaining the spatial extent of different compositional trends. Distinct spectral features that continue across clusters of pixels-r which can be correlated to topographic or morphologic features- generally indicate separate lithologic units. Furthermore, the contacts between several different lithologic units within the central peaks of a single crater may be obvious in ratio images but too subtle to detect from individual spectra. Examples of spectrally distinct, spatially coherent units within central peaks are presented in Fig. 5 for the craters Stevinus and Theophilus.

On the other hand, the advantage of evaluating individual spectra lies in mineralogical interpretation. It is tempting to look at the five- color spectra of Clementine as a series of discrete values, when in fact they represent a limited glimpse of what is actually a smoothly varying spectrum. The shape of the spectrum-as best as can be interpolated from the five points-should be critically examined as an essential test of rock type classification results (discussed below). On the basis of spatial patterns in image ratios, three to four spectra were selected to represent each distinct rock type in a crater's peaks. The spectra were always selected from directly illuminated slopes, in order to minimize albedo variations due to topography. Every spectrum is an average of a 4 x 4 pixel box, minimizing the small spatial errors associated with image compression and instrumental effects (McEwen and Robinson, 1997). These five-color spectra make up the database discussed throughout this paper.

Classification of Central Peak Spectra

Each spectrum of central peak lithologies was classified according to the mineralogy or rock type present. Although many spectra exhibit clear and distinctive mineral absorptions, consistency in classifying the composition of all central peaks spectra is difficult to maintain in a nonsubjective manner. Quantitative criteria on which to base the classification are highly desirable and two criteria are particularly appropriate here.

Key Ratio-A first-order estimate of the relative abundance of mafic minerals and plagioclase comes from the apparent strength of the 1 p m absorption band. This is typically done by taking the ratio of the 0.75 pm band to one of the bands near 1 pm. Because the mafic mineralogy determines which of the three long wavelength bands is closest to the absorption band minimum, we use whichever of the three ratios is strongest for any given spectrum and refer to this parameter as the "key ratio."

Spectral CurvatureMafic mineral type is dependent on the wavelength of the ferrous absorption band minimum near 1 p m and is difficult to detect accurately in five-color spectra. The angle formed in the Clementine five-color spectra between 0.75, 0.90, and 0.95 p m

Mineralogy of the lunar crust: Results from Clementine

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FIG. 4. Near-infrared telescopic spectra of the central peaks of Bullialdus (noritic), Copernicus (troctolitic), and Tycho (gabbroic) (from Hawke et al., 1986; Pieters, 1991), with comparisons to Clementine. (a) Full-resolution NIR spectra relative to Sun (scaled to unity near 1 pm). The dashed lines indicate the limits of the Clernentine UVVIS wavelengths. The mafic absorption bands are readily identified after continuum removal. (b) The telescopic spectra convolved through Clementine CCD filters. These examples indicate that a first-order estimate of mineralogy may be made based on the shape and strength of the mafic absorption band near 1 pm. (c) A Clementine spectrum from a 22 km box centered on the peaks of Bullialdus, compared to the NIR telescopic spectrum of the same location. The Clementine spectrum has been corrected to be equivalent to diffuse hemispherical reflectance. The comparison is presented as a qualitative validation only of the Clementine calibration. Although the fit between the two spectra is excellent is this example, there are still color-to-color errors in the Clementine data that vary

4 I Copernicus 4A: Telescopic NIR Spectra

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provides a good measure of the general curvature of the 1 p m absorption band at wavelengths critical to distinguishing among mafic minerals (e.g., see Fig. 3). Spectral curvature increases as the wavelength of the absorption band minimum moves to longer wavelengths.

The "spectral curvature" and "key ratio" parameters are designed to represent much of the mineralogical information inherent in the Clementine five spectral bands. The key ratio approximates band depth and is a proxy for the abundance of mafic minerals. Spectral curvature is generally sensitive to the type of mafic silicate present and especially to pyroxene composition.

The relation between values for spectral curvature and key ratio is shown in Fig. 6a for the principal lunar minerals (convolved through Clementine filters) whose spectra are shown in Figs. 2 and 3. Also shown are values for a series of mixtures between these minerals, calculated using the bi-directional reflectance equations of Hapke (1981) to linearize the mixing systematics (explained in detail by Mustard and Pieters, 1987). For these calculations, particle sizes

were assumed to be equal for all samples (all samples are 2250 pm) and to scatter light isotropically. Although the calculated mineral mixtures do not follow a linear trend in the plot (nor should they), they are clearly distinguishable by the combination of the key ratio and the spectral curvature parameters.

The actual central peak data define a similar relation with pure plagioclase, plagioclase + low-Ca pyroxene, and plagioclase + high- Ca pyroxene as compositional endmembers. The spectral curvature primarily distinguishes between longer and shorter wavelength absorptions due to high- and low-Ca pyroxene, whereas the key ratio allows estimates of relative plagioclase and mafic mineral abundances. Although a direct mineralogical comparison to laboratory samples is not possible given the number of uncontrolled variables, the relative position of mineral mixtures with respect to the endmembers should remain constant. In other words, if the effects of uncontrolled variables are at least similar for all the central peaks, the mixing lines defined by the laboratory spectra should be


FIG. 5. Clementine images of the central peaks of the craters Stevinus (a) and Theophilus (b) (spatial scale is the same for all images). For each crater, a brightness image (0.75 pm) is presented above and a 0.75 p m /0.95 Icm ratio image below. The ratio is related to the strength of the mafic absorption band near 1 p m where bright areas have a strong absorption. In both craters, areas of concentrated mafic material are not necessarily correlated to albedo boundaries. (a) Based on spectra from the peaks, Stevinus has four lithologic units in its peaks ("GNTA2" and three mafic units). The arrows indicate areas that have sharp albedo boundaries that are not correlated to mafic mineral abundance, as indicated in the ratio image. (b) Theophilus' peaks are largely composed of anorthosite with small amounts of troctolite. The upper arrow indicates an example of material that is bright but not mafc-rich (anorthosite). The lower arrow points out an area that is both bright and mafic-rich (troctolite).

applicable to the central peaks data but would require a correction factor for estimates of mineral proportion.

Using selected but well-defined Clementine spectra as tie points (for craters whose composition has been determined based upon telescopic measurements and are believed to approximate compositional endmembers), we found that trends in Clementine central

peak spectra follow those found in laboratory data for mineral mixtures, if the compositional endmembers are translated to larger curvatures and weaker key ratios. The shift in curvature and key ratio values is a consequence of weaker absorption bands in the Clementine data, due primarily to the general difference between spectra of laboratory mineral powders and actual rocks (Mustard et a/ . , 1993),


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FIG. 6. (a) Plot of spectral curvature vs. key ratio (0.90 pm10.75 pm for low-Ca pyroxene, 0.95 pm10 7 5 p n for high-Ca pyroxene, 1.0 pdO.75 pm for olivine and plagioclase), for the lunar minerals whose bidirectional reflectance spectra are shown in Figs. 2 and 3 , and for a series of calculated mixtures between them. The mixtures are shown in 10% increment.. (b) Plot of spectral curvature vs. key ratio for the central peak lithologies. The dashed lines are defined by the minerals plotted in (a). They have been shilled linearly (as discussed in the text) but not scaled in any way in order to provide systematic estimates of mineral abundances from the Clementine data. The shift in values (towards larger spectral curvatures and higher key ratios) is due to weaker absorption bands in the Clementine data, caused by the general difference between spectra of laboratory mineral powders and actual rocks (Mustard et al., 1993), and probably to some limited soil development as well. An estimate of error due to instrument precision is provided in the upper left comer.

and probably to some limited soil development as well. This trans- lation allows the quantitative mineral abundances defined by Hapke modeling of the laboratory endmembers to be used to assign approximate abundances to the central peaks data grouped into lithologic categories, as shown in Fig. 6b. A series of Clementine central peaks spectra that illustrate representative central peak lithologies-based on the mineralogical abundance estimates illustrated in Fig. 6 b a r e shown in Fig. 7. Each of these rock types are discussed more thoroughly below. It should be noted that this procedure to quantify the mineral abundances in observed rock types is currently our best

estimate within available data. Because many of the conclusions in this analysis depend on this assessment, it is very important for future missions to validate this procedure when high-spectral resolution data become available.

Assignment of olivine-rich lithologies required special handling. The primary difference between olivine and high-Ca pyroxene absorption bands for these five-color data is in the inflection between 0.95 and 1.0 p m (rather than between 0.90 and 0.95 pm). The spectral curvature does not distinguish well between these minerals. For example, a pure high-Ca pyroxene could have the same spectral curvature value as a mixture of olivine and low- Ca pyroxene. Therefore, in the initial classification, every- thing with a large spectral curvature was grouped together. Then the troctolitic rocks were separated from the gabbroic rocks based on visual inspection of the spectra near 0.95-1.00 pm. This step in the compositional classification is one of the most vulnerable to the assumption of similar maturity among the central peaks. Soil development leads to apparently weaker absorption bands and apparently larger spectral curvatures.

Brightness is an important compositional parameter in lunar rocks, and one which is not easily taken into account by the mineralogical classification using spectral curvature and key ratio. Nevertheless, on a local scale, brightness can be an important distinguishing factor in the identification of distinct lithologies. If space weathering and topographic effects can be ruled out as causes of a variation in brightness, then either composition (plagioclase abundance, presence of opaque minerals, etc.) or grain size are typically responsible. Reflectance spectra do not distinguish between grain size and compositional effects; however, we felt that a change in albedo due to either cause justified classifying a unit as a distinct lithologic unit within a crater.

It is difficult to quantify absolute uncertainties in this approach to mineral classification. For example, with any individual crater, small local calibration errors (e.g., due to scattered light) could conceivably affect the five- color spectra in a way that would cause it to be misclassi- fied. For this reason, none of the general conclusions from our analysis depend on interpretations for an individual crater. On the other hand, given the very regular and consistent variation of spectra for the statistically significant number of craters in this analysis (109), the patterns identified have a high degree of reliability.

Central Peaks Lithologies

Using the classification methods presented above, we have confidence in the relative mineralogical abundance

estimates for the Clementine central peaks spectra. These relative abundances are linked to the petrographic classification scheme for lunar highland rocks developed by Stomer el al. (1980) that distinguishes rock types by the amount of plagioclase and the type and abundance of mafic minerals. According to the Stomer et al. scheme, anorthosites have >90% plagioclase. Mafic-bearing rock types contain plagioclase with various amounts of low-Ca pyroxene (noritic anorthosite, anorthositic norite, norite), high-Ca pyroxene (gabbroic anorthosite, anorthositic gabbro, gabbro), or olivine (troctolitic anorthosite, anorthositic troctolite, troctolite).


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FIG. 7. Example spectra for each central peaks lithologic category. These data have been normalized to 0.75 p m to emphasize the shape and strength of the mafic absorption band, For spectra of rocks that are believed to be comparable to noritic anorthosites, gabbroic anorthosites, and troctolitic anorthosites, mafic mineralogy is ambiguous and is therefore grouped into two general categories correlated to plagioclase abundance ("GNTAl " and "GNTAZ").

We have used the Stoffler el a/. classification scheme as a general framework for classifying the Clementine central peak lithologies. For the purposes of this work, all lithologies are stated in terms of highland rock types. The Clementine classification does not allow the same level of detail prescribed in the Stoffler et a/. system, but the compositional classes are broadly compatible as illustrated in Fig. 8. It should be noted that the classification of spectra is some- what simplistic and relies heavily on the key ratiokpectral curvature scheme described above and the assumption of comparable maturity values for the peaks. Within these limits, however, the data provide a useful first-order assessment of mineralogy of the lunar crust between 5 and 20 km depth.

Anorthosites-Anorthosites (labeled "A" in our classification scheme) have no detectable mafic minerals and are comparable to those traditionally identified as anorthosites in NIR spectra (Hawke et al., 1992b, 1995; Pieters et al., 1996a). Because reflectance spectra are sensitive to very small amounts of mafic minerals in an anorthositic rock, "A" rocks are believed to contain 4 0 % mafic minerals (Pieters, 1986). Many of the "A" spectra have a down- ward inflection at 1 .O pm, which suggests the presence of a 1.3 p m plagioclase absorption band, indicating a component that has not been severely shocked.

"GNTA1" and "GNTAZ" Rocks-Spectra that fall within the 8&90% anorthosite range in the plot in Fig. 6b appear to fall into two clusters, breaking naturally near 85% plagioclase. All of the spectra contain a small amount of detectable mafic material, but those that fall in the 8&85% plagioclase range have visibly stronger absorption bands. However, the bands are usually not sufficiently developed to allow the mafic mineralogy to be identified. Furthermore, the variations in spectral curvature for rocks in this category approach the I% accuracy level estimated for Clementine color differences (McEwen and Robinson, 1997). Therefore, no attempt is made to classify these rocks by mafic mineral type, and the central peaks lithologies are combined into a general category that we refer to as gabbroic-noritic- troctolitic anorthosite ("GNTA"). We distinguish between "GNTAl" (85-90% plagioclase) and "GNTA2" (80-85% plagioclase) based upon spectral characteristics only, as there is no relevant lithologic distinction in the Stoffler et al. classification (Fig. 8).

It is important to note that any instances where our assumption of similar peak maturity fails leads to an overestimation of the number of GNTA rocks. Because the GNTA spectra have weak absorption bands, they could indicate either ( I ) rocks with a low abundance of mafic minerals (as we identify them) or (2 ) the result of optical alteration due to soil development. More mature surfaces, with a higher mafic mineral content than classified here, could fall into this class if small areas on the central peaks have slopes sufficiently shallow to build pockets of mature soils (easily below the spatial resolution of the Clementine data), weakening the mafic absorption. If present, the effect of mature soils would nevertheless be greatest for rocks with a relatively low mafic mineral abundance because all soils are mixtures. Therefore, rocks in the GNTA category have the highest uncertainty associated with their classification, but they are likely to be very anorthositic.

Gabbroic and Noritic Rocks-As depicted in Fig. 8, the classification for gabbroic-noritic rocks is based on the ternary system of plagioclase, orthopyroxene, and clinopyroxene. A first- order classification can be based solely on the type of pyroxene identified, placing a material in the noritic (opx > cpx) half of the triangle or the gabbroic (cpx > opx) half. If the mafic absorption is relatively strong (indicating >20% mafic minerals) with Clementine spectra, the shape of a spectrum (with either high-Ca or low-Ca pyroxene as the primary mafic mineral) is readily identified and separated using spectral curvature (Figs. 4b and 7b). Mixtures of the two are defined here as "gabbronorites," meaning that they exhibit an intermediate mafic mineral absorption that could include both high- and low-Ca pyroxene. Because the limited spectral coverage of the Clementine UVVIS multispectral data does not allow estimation of continuum effects, noritic properties are easily underestimated for weak bands.

Troctolitic Rocks-Olivine-bearing rocks are classified similarly with respect to mixtures with plagioclase. Mixtures of olivine and pyroxene, however, are extremely difficult to characterize accurately, even in data with high-spectral resolution (Singer, 1981). Olivine must make up -3&50% of the mafic mineral content to be detected by the shape of the 1 p m absorption (Cloutis et a/., 1986). The characteristic signature of olivine is nevertheless clearly seen in several Clernentine spectra (Pieters et al., 1996a; Pieters and Tompkins, unpubl. data, 1998). When olivine is unambiguously identified, the rock in question is therefore either a troctolite or a dunite. The fact that pyroxene-olivine mixtures are difficult to remotely detect obviously does not mean that they do not exist. It is possible that some central peaks included in this survey have more olivine- bearing lithologies present than the results indicate.


Pla g ioc I as e 1. Anorthosite A - Anorthosite 2. Noritic Anorthosite

4. Anorthositic Norite

6. Norite 7. Gabbroic Norite 8. Noritic Gabbro 9. Gabbro

10. Pyroxenite

GNTA1 - Madic-tAnorthosite GNTAS - MadiciAnorthosite AN - Anorthositic Norite AGN - Anorthositic

Gsbbro n or it e AG - Anorthositic Gsbbro N - Norite UU - Gabbronorite G - cBbbro

Ort hopyroxene Clinopy roxene FIG. 8. Ternary classification diagram of the plagioclase-orthopyroxene-clinopyroxene system illustrating the lunar highland rock classification scheme of Stoffler et al. (1980) for those rocks. The classification for olivine-bearing rocks is comparable-with troctolite, anorthositic troctolite, and troctolitic anorthosite equivalent to gabbro (or norite), anorthositic gabbro (or norite), and gabbroic (or noritic) anorthosite. The smaller ternary plot illustrates the Clementine classification for the pyroxene-rich rocks and its relationship to the Stoffler diagram (olivine-rich rocks are classified similarly; but as discussed in the text, mixtures of olivine and pyroxene cannot be identified from these data). The relative abundance estimates allow spectrally distinct units on the Moon to be translated to possible rock types, Note that the Clementine anorthosites are subdivided into two categories and that along the pyroxene mixing line, three categories are identified rather than four.

OBSERVATIONS AND RESULTS A global summary of central peak lithologies is presented in

map form in Fig. 9. Rock types derived from the Clementine data are superimposed on a map of crustal thickness (from Zuber et al., 1994), with major basins and maria outlined in white for context. Multiple lithologies found at the same crater are indicated by super- position of symbols and colors. Thus, this map of crustal rock types indicates the presence-but not the abundant-f different rock types within each set of peaks. For reference, all the surveyed craters, their locations, diameters, inferred compositions, and relevant Clementine filenames are listed in Table 1.

The variety of rock types shown in Fig. 9 indicates a lunar crust that is compositionally diverse at all spatial scales. Central peak composition varies extensively between craters, but globally consistent patterns exist that may be correlated to topography, crustal thickness, and geologic setting. Locally (-500-5000 m) the central peaks are diverse, containing multiple lithologies at -40% of the craters, where the smallest spatially coherent units extend 4-5 pixels (2100 dpixel). These observations are discussed in more detail below-in terms of global, regional, and local compositional variability, both laterally and (where possible) vertically.

Global and Regional Trends The surveyed craters have been subdivided by geologic setting

into two general categories: those in the highlands and those within major impact basins. The distinction was made because highland and basin-related craters effectively sample completely different levels of the crust. Craters interior to basins were formed on a surface from which kilometers worth of crustal material have been removed during basin formation. Geophysical calculations of crustal thickness suggest typical differences in crustal thickness of -20 km between basin interiors and the lunar highlands (e.g., Neumann et al., 1996) and further suggest a global discontinuity in the highland crust itself at -20 km depth that is compositionally derived (Wieczorek and

Phillips, 1997). Although some material can be added back to the basins (via mare volcanism, or the deposition of melt or ejecta from subsequent craters and basins), to the first order the craters interior to impact basins are sampling deeper crust. On the other hand, craters in the highlands are likely to sample upper crust. Given the range of crater diameters surveyed, the highland craters are unlikely to sample more than the upper 30 km of local crust.

Central peak lithologies associated with these two geologic set- tings are summarized in Table 2, which treats the discrete lithologies identified among the central peaks as a sort of "sample collection" in their own right. From these data, several compositional trends are apparent.

(1) Central peaks containing anorthosite are abundant across the Moon and found in 62% of all the craters surveyed; approximately one-fourth of those craters contain anorthosite alone, without any other rock types apparent in the central peaks.

(2) Anorthosite-bearing peaks occur more frequently in the highlands than in or near basins. The highland central peaks consist primarily of anorthosite and GNTAl rocks (85-90% plagioclase), although more mafic rocks do occur infrequently. Given a central peak "sample collection" comprising all of the discrete rock types identified in the survey (see Table Z), the highlands can be estimated to have a plagioclase content of -82%.

(3) The GNTAl rock type is equally distributed between highlands and basins.

(4) Central peaks within basins are dominated by GNTAI, GNTA2 (80-85% plagioclase), and anorthositic norite. Specifically, craters whose peaks contain GNTA2 are twice as common among basins as in the highlands, whereas peaks containing anorthositic norite are six times as common. The bulk composition of central peaks within basins corresponds to a plagioclase content Of -74%.

(5) Unlike anorthositic norite, the other mafic-bearing rock types (anorthositic gabbro, anorthositic gabbronorite, and anorthositic troctolite) are apparently equally distributed among basins and


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FIG. 9. Map of central peaks lithologies superimposed on the crustal thickness model of Zuber et al. (1994). Symbols and colors for non-anorthositic rocks were selected such that a single color is correlated to each identifiable mafk mineral (olivine, high- and Iow-Ca pyroxene, and an intermediate pyroxene composition), and that the intensity of the color-as well as the size of the symbol-increase with increasing mafic mineral abundance. This scheme indicates the presence, but not the abundance, of multiple lithologies within each crater’s central peaks. Global and regional trends in composition are discussed in the text.

highlands. However, the sample size is small, and the results potentially deceptive. In all, these rocks make up -1 1% of the lithologies identified among the central peaks.

(6) With one exception, the most mafic central peaks (those containing norite, gabbronorite, gabbro, and troctolite) only occur within basins. The exception is Tycho, whose unique composition is discussed in detail (Hawke et al., 1986; Lucey and Hawke, 1989; Pieters, 1993). Given the small number of samples with a high abundance of mafic minerals, it is not clear that any mafic mineral type occurs more frequently than others. Overall, the most mafic rocks make up <5% (4.7%) of the assemblage of discrete rock types identified in the central peaks survey.

Although these observations can be associated to the first order with depth in the crust (Le. , highlands vs. basins indicating upper vs. lower crust), the craters provide a more direct means to sample vertical compositional variability. A simple representation of the relationships (or lack thereof) between central peak composition and crater size is presented in Fig. 10. Crater diameters are used as a proxy for the depth at which central peaks originated ( i e . , larger

diameters imply deeper origins) and the key ratio distinguishes between feldspathic and mafic lithologies (key ratio varies inversely with increasing mafic content). There is no immediately apparent correlation between composition and depth of origin. Because each crater has impacted into regions of crust whose surface height has been altered by events-such as basin excavation, volcanism, and impact melt formation-the apparently random distribution is not surprising. Even in the highlands, added material from basin ejecta could mask any correlation between composition and depth. More detailed studies of the relationship between depth and composition- which estimate the depth of central peak origin from the base of the crust rather than the surface (and test a range of crater scaling relationships and crustal thickness estimatespare underway and will be reported in future work.

Despite such general limitations of the data, it is nonetheless evident from Fig. 10 that many of the lithologic classes occur at all depths. In particular, compositions with key ratios larger than -0.92 (i.e., the more feldspathic lithologies) are found in the central peaks of craters of all diameters, both within basins and in the highlands.


TABLE 2. Abundance and distribution of central peaks lithologies, with respect to a first-order description of geologic setting.

Number of occurrences of discrete lithologic units

lands lands

Percentage of craters containing each rock type

Rock type found in peaks: High- Basins Total High- Basins Total

Anorthosite (alone) Anorthosite (with other

GNTAl GNTA2 Anorthositic norite Anorthositic gabbronorite Anorthositic gabbro Anorthositic troctolite Norite Gabbronorite Gabbro Troctolite Total craters Total discrete units

rock types)

12 36

39 22 3 6 5 3 0 0 1 0

65 127

7 19 13 49

25 64 25 47 13 16

5 1 1 4 9 2 5 3 3 4 4 2 3 1 1

44 109 104 231

18.5 15.9 17.4 55.4 29.5 45.0

60.0 56.8 58.7 33.8 56.8 43.1 4.6 29.5 14.7 9.2 11.4 10.1 7.7 9.1 8.3 4.6 4.5 4.6 0.0 6.8 2.8 0.0 9.1 3.7 1.5 4.5 2.8 0.0 2.3 0.9

Craters occurring interior to impact basins are distinguished from highland craters

Local Heterogeneity

Although most of the discussion presented here focuses on trends among central peaks, complex trends within central peaks are worth noting as well. Many central peaks (-40%) contain multiple lithologies (Table 1 lists the number and types at each crater). In general, craters found within impact basins are more heterogeneous (Le., their central peaks more frequently exhibit multiple lithologies).

The nearside craters Stevinus (just south of Mare Fecunditatis) and Theophilus (along a ring of Nectaris) provide representative examples of compositional diversity within an individual set of peaks (Fig. 5). Three distinct, relatively mafic units are visible at Stevinus with sharp contacts showing variations in albedo and color. At Theophilus, the central peaks contain regions of both anorthositic and troctolitic composition. Previous studies based on telescopic point spectra of Theophilus's peaks detected only anorthosite (e.g., Pieters, 1986), either because the spatial scale of the telescopic spectrum was too large for small amounts of troctolite to be detected or (more likely) because the telescope aperture simply missed the troctolitic areas. The high-spatial resolution of the Clementine data demonstrate that the peaks in fact contain small distinctive areas of troctolite as well as anorthosite. Similarly, even higher spatial resolution data may reveal that some of the "discrete" rock types identified in this survey are in fact combinations of smaller, lithologically distinct units.

Although a majority of the central peaks exhibit multiple lithologies (Table I), the style of variability is not constant among them. For example, for many of the central peaks, the different compositions appear to vary smoothly from one rock type to another (e.g., noritic anorthosite to anorthositic norite) and may be co-genetic. In contrast, -20% of the central peaks have multiple lithologies separated by distinct geologic contacts, which appear to have been brought together by intrusion or faulting (of which Stevinus' peaks are an example). The multiple lithologies within these central peaks provide clues to local variability in crustal rocks and are closely linked to local geologic history. Detailed studies of several craters are underway and will be reported in future work.

DISCUSSION

The central peaks data relies on the excellent foundation of near-infrared telescopic studies (which document the occurrence of highland rock types across the lunar nearside) by extending and adding new data points, by spatially resolving previously studied areas, and by including farside data. Based on the observations detailed above, as well as previous observations and predictions of crustal composition, we examine the composition and structure of the Moon's crust. The discussion is governed by the assumption that highland craters have sampled upper crust (-5-30 km depth) and craters within basins have sampled lower crust (-25-55 km). The depth ranges are rough estimates only.

Upper Crust (Highlands)

The composition of the upper crust based on the central peaks survey suggests that the upper crust is dominated by anorthosite and GNTAl. Together, these compositions (representing lithologies containing >85% plagioclase) are found at -90% of the highland craters sampled (see Table I). All of the remaining craters contain some GNTA2 (80-85% plagioclase). Assuming that

each rock type contains the minimum amount of plagioclase as depicted in Fig. 8, the bulk plagioclase content of the highland central peaks-and thus presumably the upper crust of the Moon-is -82%. This value is within range of estimates of an A1203 content of 27-29 wt%~ derived for soils by Lucey et al. ( 1 998)-also based upon Clementine data-and slightly higher than geochemically based estimates of 24-27 wt%~ (e.g. , Warren, 1990). The high plagioclase values and the apparent abundance of mafic-free (or "pure" anorthosite) at 5-30 krn depth across the Moon further supports previous studies of surface composition (e.g. , Hawke et al., 1992% 1995; Lucey et al., 1995; Pieters, 1993; Spudis and Davis, 1986) in requiring a magma ocean to account for the globally pervasive anorthositic crust that is observed, rather than serial magmatism, which predicts a significantly lower plagioclase abundance (Walker, 1983).

Given the anorthositic nature of the upper crust, the rare craters that contain more mafic lithologies bear closer examination. The fact that Mg-suite rocks are so abundant among the pristine lunar samples suggests that many of the mafic central peaks represent part of the Mg-suite. The most commonly accepted explanation for the Mg-suite rocks is that they formed as mafic intrusions into the crust (e.g. , James, 1980; Warren and Wasson, 1980). If the depths at which Mg-suite lunar samples crystallized were between 10-30 km depth (suggested by Herzberg and Baker, 1980), then such plutons in the lunar highlands could be sampled directly by the craters in the size range of this survey, and the relative abundance of detection should mimic the abundance within the crust.

There are six highland craters that contain only rock types more mafic than "GNTA2;" these occur among craters ranging from 45- 110 km in diameter (some are not shown in Fig. 10 because they occur at high latitudes), implying the occurrence of very mafic-rich lithologies at a range of depths within the crust. These mafic central peaks may be indicative of mafic intrusions into the crust and may be examples of the postmagma ocean magmatism that is commonly predicted to account for the Mg-suite.

The most mafic highland crater among those surveyed is Tycho, with a composition ranging from gabbro to gabbronorite. This com-


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FIG. 10. Plots of crater diameter (as a proxy for depth of central peak origin) against key ratio for all the discrete lithologic units classified from the central peaks survey. (a) Craters found in the highlands. These central peaks are believed to represent primarily upper crustal rocks. (b) Craters found in basins. There are no obvious trends between diameter and composition. These central peaks have probably exhumed lower crust, but the depths that they sample are governed more strongly by the preexisting impact basin structure than by the diameter of each impact crater.

position is consistent with conclusions drawn from telescopic NIR observations by Hawke et al. (1986). Hawke et al. further showed that Tycho's walls-measured at several locations around the crater- are of similar composition to its peaks. Because the walls expose much shallower material than do the peaks, the gabbroic material at Tycho appears to extend vertically from near the surface (the wall material) to at least -15 km beneath the surface (the predicted depth of central peak origin, based on Tycho's diameter of 85 km) and laterally for distances on the scale of the crater diameter. However, it does not appear to extend far beyond Tycho, as nearby craters have purely anorthositic peaks. For at least one of the six mafic highland craters in the survey, therefore, evidence from previous remote sensing studies suggests the mafic unit to be both laterally and vertically extensive (tens of kilometers), but localized (hundreds of kilometers).

On the other hand, the Clementine troctolite data suggest that other candidate plutons are not spatially extensive at all. As explained above, olivine is identified in the Clementine five-color spectra by its lack of spectral curvature, and because the absorption band

minimum appears to be near 1 .O p n or longer wavelengths. Among the central peaks, there are six sites that have identifiable troctolitic compositions, ranging from troctolitic anorthosite to troctolite. The occurrence of troctolitic central peaks appears not to be correlated with excavation depth or location: the craters range in size from 50-180 km and occur in both highland and basin-related terrain. Despite the apparently random distribution in the crust, all of the troctolite-bearing peaks coexist with anorthosite, generally with very sharp compositional boundaries (e.g., Pieters et al., 1996a; Pieters and Tompkins, unpubl. data, 1998). If the troctolite is part of an intrusion into the anorthositic crust, then either fragments of both the pluton and the host rock have been uplifted, or both the anorthosite and the troctolite are part of a differentiated pluton. The coincidence that all of the central peaks have uplifted material from a compositional boundary between troctolite and anorthosite suggests (despite the small number of samples) that the troctolite unit is not extensive enough to comprise an entire assemblage of central peaks.

The occurrence and abundance of troctolites are of particular interest because the formation of the troctolites in the lunar sample collection have long posed an intriguing puzzle for petrologists (e.g., Hess, 1994). The Mg* values (MgO/(MgO + FeO)) of lunar troctolites are too high for olivine crystallizing from primitive melts that have reached plagioclase saturation. Hess (1 994) has suggested that the lunar troctolites must form either by differentiation of impact- melt sheets or as thermal plumes of primitive (undifferentiated) mantle entraining magnesian magma ocean cumulates. With only six locations, neither possibility can be completely ruled out. However, melt sheets are not a reasonable explanation for those craters that occur either in the highlands or whose peaks originate at great depth in the crust. Lower Crust (Basin Interiors)

Geophysical models suggest that a compositional transition (from upper to lower crust) occurs at -20 km depth (Wieczorek and Phillips, 1997). This lower crust may be sampled by some of the larger highland craters but is better represented by those craters occurring within impact basins. Craters impacting into basin interiors are likely to derive their central peaks from a thinned crust, because upper crust has been removed by the basin-forming event. Although the actual amount of crust removed varies between basins, it is reasonable to expect that lower crustal material is near the surface within most of the major basins, despite blanketing layers of impact melt, ejecta, and mare basalt. Although those blanketing layers are themselves among the materials potentially exhumed by the central peaks, the estimated depth of origin for most of the peaks argues against such a scenario. To be sampled by the central peaks in this survey, a basalt layer and a basin-melt sheet (as well as any overlying ejecta) must be >5 km thick for even the smallest craters in the survey. In most basins, the central peaks are more likely to have exhumed material from beneath these layers. This assumption is substantiated by two observations. First, none of the surveyed craters that have impacted into terrains with mare basalt on the surface exhibit spectral characteristics indicative of basalt in their peaks, although basalt is likely to be present along crater walls and rims (Pieters et al., 1994; Pieters and Tompkins, unpubl. data, 1998). Second, the heterogeneity of many of the central peaks interior to basins argues against their having exhumed a homogenized layer of impact melt.

Although the above arguments hold for most of the impact basins on the Moon, it is still possible among the larger basins that melt sheets are thick enough to be included in the central peaks of craters. For example, the enormous SPA Basin may have formed indeed a melt sheet thick enough to be sampled by the craters in this survey.


Those craters interior to SPA (both large and small) exhibit a remarkably small compositional range, with low-Ca pyroxene the only mafic mineral evident. The uniform mineralogy and potential origins of SPA basin rocks are discussed in detail by Pieters et al. (1997), who conclude that a well-mixed melt sheet ( i e . , not differentiated) is a viable source of the material exposed among central peaks of SPA impact craters.

Despite the uncertainties associated with impact melt, the lower crust has a composition that is clearly dominated by "GNTAI" through anorthositic norite (Table 2), which is consistent with previous remote sensing and geochemical studies (e.g., Hawke et al., 1991, 1993; Spudis, 1993; Spudis and Davis, 1986; Spudis et al., 1984, 1988, 1989). When anorthosite does occur, it is typically found near basin rims and rings (Fig. 9), verifying and extending previous observations of abundant anorthosite associated with basin rings (e.g., Hawke et al., 1995; Peterson et af., 1995, 1996; Pieters, 1986; Spudis et al., 1994, Hawke, 1993), which are believed to originate within the upper half of the crust. Overall, the central peaks results suggest the average composition of the lower crust to be extremely anorthositic, with a plagioclase content of -75% (71% if all anorthosite is assumed to be upper crustal in origin), corresponding to an approximate A1203 content of 25-27 wtO/. Lower crustal composition has been geochemically modeled to be relatively mafic, comparable to LKFM (Ryder and Wood, 1977) (with -18 wtV0 A1203); however, the central peaks results suggest that the lower crust is significantly more anorthositic.

The "GNTAI " and "GNTA2" are interpreted to be breccias that represent the gradual increase in mafic content with depth in the crust, continuing the trend observed among highland central peaks. The abundances for each rock type as shown in Table 2 indicate a crust whose composition varies from pure anorthosite near the surface, through "GNTAI " (which spans both highlands and basins equally) to "GNTA2" in the lower crust. The increase in mafic content appears to continue to anorthositic norite at the greatest depths sampled in this survey, which is found in 30% of the basin-related craters. However, it is also possible that the anorthositic norite peaks are intrusive in origin and thus are candidate Mg-suite rocks, rather than a product of a magma ocean.

The central peaks compositions also suggest the lower crust to be more heterogeneous than the upper crust, exhibiting more lithologic variability within individual craters, and also containing the majority of the most mafic rocks identified among the central peaks (Fig. 9; Tables 1 and 2). In addition to anorthositic norite, every other rock type defined in Fig. 8 is represented among the subset of craters that sample lower crust. These lithologies are all potential Mg-suite rocks: plutons that either formed at sufficient depth within the crust that basin-scale impacts were required to expose them or else formed subsequent to basin formation. If any of the basins predate the plutons, basin formation and subsequent fracturing of the crust may have made intrusion easier. Although the Mg-suite rocks are generally believed to be younger than the anorthosites, their ages relative to potentially early basin formation are not known well enough to support or deny such a scenario.

Bulk Crust

The nonmare crust as a whole (interpreted from the central peaks survey) is remarkably anorthositic, with only nine craters having lithologies that contain <60% plagioclase. There are an additional 25 craters whose most mafic units have between 60 and 80% plagioclase. The bulk plagioclase content for the entire assemblage of central

peaks lithologies is -81%. This is higher than many previous estimates of bulk crustal composition, which range from -55 to 70% plagioclase (e.g., Ryder and Wood, 1977; Spudis and Davis, 1986). The Clementine-derived compositions-from other studies (Lucey et al., 1995, 1998) as well as this one-suggest that the assumptions of crustal composition that have been used in the past to constrain mass-balance calculations may be significantly overestimating the mafic content of the crust. For example, Warren and Kallemeyn (1993) have predicted -30 wt% of the lunar crust to have been em- placed by Mg-suite magmatism, based upon estimates of ferroan anorthosite and nonferroan anorthosite composition that are ultimately derived from the lunar sample collection. A higher anorthositic composition for both the bulk and lower crust (as suggested by this study and supported by estimates of surface composition from Lucey et af., 1995, 1998) in Warren and Kallemeyn's predictions leads to lower values of Mg-suite magmatism.

SUMMARY AND FUTURE QUESTIONS The central peaks of 109 impact craters have been analyzed with

Clementine multispectral images in order to determine their mineral compositions. Distinct rock types are identified which are believed to represent crustal rocks uplifted from 5-30 krn beneath the surface, including both upper and lower crustal rocks depending on whether craters have impacted into highlands or basins. From these results, it is evident that the lunar crust is compositionally diverse, both globally and at local 100 m scales found within individual sets of central peaks, but is generally consistent with previous models of crustal structure and composition. More specifically, the following conclusions are drawn:

(1) The lunar crust is extremely anorthositic, with a plagioclase content (based on the central peaks surveys alone) of -82% for the upper crust (-5-30 km) and -75% for the lower crust (-25-55 km), and -81% for the entire assemblage of lithologies identified among the central peaks. The lower crust in particular is less mafic than previously assumed and is not consistent with an LKFM composition.

(2) Crustal composition gradually increases in mafic content with depth from predominantly "pure" anorthosite, through "GNTAI " and "GNTA2" (85-90% and 8&85% plagioclase respectively), possibly to anorthositic norite.

(3) Candidate mafic plutons occur in both highlands and basins. At least one possible gabbroic pluton (at Tycho) has been demonstrated through telescopic NIR data (Hawke et af., 1986) to be vertically and laterally massive and generally monomineralic. Candidate troctolitic plutons that appear in the Clementine data consist of distinct layers of troctolitic rock adjacent to anorthosite and may be examples of differentiated plutons.

(4) Mafic rocks are rare among the surveyed craters, with only -8% having a plagioclase content <6O%, and 31% (including the previous 8%) with a plagioclase content of 6040%. The mafic lithologies identified among the central peaks are typically more anorthositic than the average value for large Mg-suite samples (Warren, 1990).

(5) The lower crust is more compositionally diverse than the highlands (at a finer spatial scale) and contains the majority of most mafic lithologies identified in the survey. This observation implies that mafic plutons-if that is what the mafic lithologies represent- form either at greater depths in the crust or form preferentially as a result of basin formation.

These conclusions are based primarily on interpretation of the UVVIS camera's five spectral channels. With the Clementine NIR


(five-color) camera data, for which calibration is underway (Lucey et al., 1997), the mineralogy could be better or more confidently determined. For example, mixtures o f pyroxene and olivine and olivine alone might be more readily identified, though only qualitatively. Accurate mineralogical abundances may be calculated through quantitative deconvolution of reflectance spectra (Mustard and Pieters, 1987; Sunshine and Pieters, 1993; Sunshine et al., 1990), which requires high precision and spectral resolution sufficient to distinguish subtle variations in absorption band shape. Hopefully, future lunar missions will provide such high-spectral resolution data. In the mean- time, the Lunar Prospector mission will also return geochemical information that will allow some of the conclusions of this paper to be tested and refined, and ultimately piece together more of the lunar crust’s complex history

Acknowledgments-Thoughtful and constructive reviews by B. Ray Hawke and Paul Lucey have greatly strengthened this paper. We also gratefully acknowledge informal but detailed coinments by Brad Jolliff, which added considerably to the manuscript’s clarity and completeness. This research was supported by NASA grants NASW-98011 (S. Tompkins), NAGW-4910 and NAGS-4366 (C. M. Pieters).

Editorial handling: R. A. F. Grieve

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